Discrete Multitone Modulation (DMT), also known as multicarrier modulation or Orthogonal Frequency-Division Multiplexing (OFDM) has been proposed in recent years for applications of transmission of data over severely distorted channels. For instance, this transmission technique has been adopted by ANSI T1E1.4 as the standard technique for the ADSL application.
A multicarrier transceiver divides the available channel bandwidth into a plurality of relatively narrow-band subchannels. These channels are equally spaced across the system bandwidth. Data are transmitted over each subchannel independently of the others. When the transmitter obeys some suitable orthogonal conditions and thus avoids interference between adjacent channels, and the subchannels are sufficiently narrow, then Inter Symbol Interference (ISI) across the subchannel bandwidth is significantly reduced. These conditions date back to R. W. Chang, "Synthesis of band-limited orthogonal signals for multi-channel data transmission," Bell Syst. Tech. Journal, pp. 1775-1796, December 1966. The DMT transmitter can tune the data rate and the transmission power over the individual subchannels according to their transmission characteristics and hence to optimize the system performance.
The transmitted data in a multi-carrier system is grouped in so-called symbols. Each symbol represents a defined time-period and a total number of bits. The bits within a symbol are allocated to the different subchannels. The bits being conveyed over each subchannel are modulated generating a so-called frequency domain vector. This vector includes coordinate sequences representing the constellation point that corresponds to each channel. The coordinates may be drawn for example from a QAM constellation.
The DMT signal is generated by means of a vector frequency-to-time transformation on the plurality of independent frequency-domain coordinates. The common transformation used is the Inverse Fast Fourier Transform (IFFT). Each entry of the IFFT sequence is a sum with a constant phasor shift of all the frequency-domain coordinates. Consequently, the resulting time-domain signal exhibits a high peak-to-average ratio.
This fact in turn has a major impact on the design requirements of the analog circuitry of the DMT transmitter. Statistically, the large amplitude spikes are very rare and in a practical transceiver, when the signal amplitude exceeds the maximum input value of a D/A converter, the output signal is clipped to this threshold level. This operation results in a distorted transmitted signal and it degrades the system performance. Several methods are now described which are regarded as reducing the probability of clipping events in DMT-based systems.
One possible technique to reduce the peak-to-average ratio is to employ pre-coding of the input data. This method requires an appreciable increase of the transmitted line rate and consequently it reduces the energy per data bit and deteriorates performance.
Another method to reduce the peak-to-average ratio is to decrease the amplitude of the entire sequence of samples corresponding to a symbol by a predetermined factor when at least one sample in this sequence is clipped. This technique requires only a few additional bits per symbol to convey the information regarding the "reduction factor" used. Evidently this increase is insignificant. However, the reduction of the transmit power increases the vulnerability of the system to noise and thus it degrades the system performance.
A third method which was proposed very recently employs a properly chosen phasor transformations of the QAM modulated carriers and a plurality of IFFT operations. This method does not degrade the system performance but it entails a significant additional complexity of the DMT transceiver since it accommodates several IFFT computations per symbol period.
The three methods mentioned above are described in detail in references 1 and 2, 3, and 4, respectively:
1. "Block coding scheme for reduction of peak to mean envelope power ratio of multicarrier transmission schemes", A. E. Jones et al., Electronics letters, Vol. 30, No. 25, pp. 2098-2099, Dec. 8th 1994. PA0 2. "Simple coding scheme to reduce peak factor in QPSK multicarrier modulation", S. J. Shepherd et al, Electronics Letters, Vol. 31, No. 14, pp. 1131-1132, Jul. 6th, 1995. PA0 3. U.S. Pat. No. 5,623,513 to Chow et al, and PA0 4. "A method to reduce the probability of clipping in DMT-based transceivers", D. J. G. Mestdagh and P. M. P. Spruyt, IEEE Transactions on Communications, Vol. 44, No. 10, pp. 1234-1238, October 1996.
The disclosures of all publications mentioned in the specification and of the publications cited therein are hereby incorporated by reference.